In recent years, in response to strong social demands for low fuel consumption of vehicles, weight reduction of tires has been sought. Among tire members, weight reduction and the like of an inner liner also have begun. The inner liner is disposed inwardly in a tire, and has a function of reducing an amount of leakage of air from inside to outside of the pneumatic tire.
Currently, a rubber composition for such an inner liner employs, for example, a rubber blend mainly containing a butyl rubber. The rubber blend contains the butyl rubber by 70 mass % to 100 mass %, and a natural rubber by 30 mass % to 0 mass %. In this way, the tire is provided with improved air permeability resistance. In addition to butylene, the rubber blend mainly containing the butyl rubber contains isoprene by approximately 1 mass %, which acts with sulfur, vulcanization accelerator, and zinc white to achieve cross-linking between rubber molecules. In the case of normal blend, the above-described butyl-based rubber needs to have a thickness of 0.6 mm to 1.0 mm for a tire of a passenger car, and needs to have a thickness of approximately 1.0 mm to 2.0 mm for a tire of a truck/bus. In order to achieve weight reduction of such tires, use of polymer, which is more excellent in air permeability resistance than the butyl-based rubber and can provide an inner liner layer with a thinner thickness, has been requested.
Conventionally, in order to achieve weight reduction of a tire, it has been proposed to use a film made of a material including a thermoplastic resin, instead of the above-described rubber composition. However, when a tire is manufactured using an inner liner of thin thermoplastic resin, the inner liner partially becomes too thin due to pressure in a vulcanization step, with the result that the finishing gauge of the inner liner in the resulting tire product becomes thinner than the designed gauge. In the thin portion of the inner liner thus finished, a phenomenon (open thread) in which a carcass cord looks to stand out takes place, thus giving a user an impression of bad appearance. In addition, when the inner liner is thin, gas barrier property becomes partially bad to decrease tire internal pressure, with the result that the tire may burst in the worst case.
Meanwhile, during traveling with the tire, large shear strain acts on a vicinity of a shoulder portion in the inner liner. When the material including the thermoplastic resin is used as the inner liner, this shear strain is likely to cause detachment at an adhesion interface between the inner liner and the carcass ply, with the result that air leakage takes place from the tire, disadvantageously.
Meanwhile, in order to achieve weight reduction of the inner liner, a technique also has been proposed to employ a thermoplastic elastomer material. However, it is known that a material, which is made thinner in thickness than the inner liner of butyl rubber and exhibits high air permeability resistance, is inferior to the inner liner of butyl rubber in terms of vulcanization adhesion strength with an insulation rubber or a carcass ply rubber adjacent to the inner liner.
When the inner liner has low vulcanization adhesion strength, air enters between the inner liner and the insulation rubber or the carcass rubber, thus resulting in a so-called “air-in phenomenon”, in which small balloon-like objects appear. The multiplicity of such small spots in the tire give a user an impression of bad appearance. In addition, during traveling, the air causes detachment to result in cracks in the inner liner. Accordingly, the tire internal pressure is decreased, with the result that the tire may burst in the worst case.
Patent Literature 1 (Japanese Patent Laying-Open No. 2010-13646) proposes to improve strength in adhesion using a petroleum resin or a terpene resin in SIBS, which is a thermoplastic elastomer, as a tackifier. However, a polyamide-based polymer is blended in addition to the SIBS, so that flection cracking resistance is decreased, disadvantageously.
Meanwhile, Patent Literature 2 (Japanese Patent Laying-Open No. 2010-100675) proposes to improve adhesive property of a carcass ply rubber using, as a tackifier, a natural rosin, terpene, a chroman indene resin, a petroleum resin, an alkylphenol resin, or the like in a blended material having a polymer capable of cross-linking with SIBS by sulfur.
However, when the polymer capable of cross-linking by sulfur is contained by not more than 100 part by weight in a technique in which the polymer capable of vulcanization by sulfur is blended by 10 part by weight to 300 part by weight relative to 100 part by weight of the SIBS, the SIBS becomes a matrix (sea portion) and the polymer capable of cross-linking by sulfur becomes a domain structure (island portion). Accordingly, adhesion strength is not improved in a contact interface with the carcass rubber. Further, when the polymer capable of cross-linking by sulfur is not less than 100 part by weight, gas barrier property is decreased in rubbers other than the butyl rubber and adhesion strength is decreased in the butyl rubber. Moreover, depending on a polymer to be blended, viscosity becomes high, with the result that a film having a thickness of 600 μm or less cannot be fabricated, disadvantageously.
In Patent Literature 3 (W02008-029781), a tire is manufactured using strips of film layer stack in which a thermoplastic resin and a thermoplastic elastomer are blended. With the film layer stack, gas barrier property and adhesive property can be improved, whereby bonding can be achieved between the ribbon-shaped strips. However, in this technique, gauge is constant in a non-vulcanized raw cover of film layer stack. Hence, when the gauge is thinned, a buttress portion or the like in the finished tire after vulcanization may become thin.